The present invention relates to electrical circuits for heaters and in particular, relates to the heating circuit of a heater where a fan motor is controlled by the current used to power the heating element.
Portable heaters are commonly used to provide supplemental heat for a localized area or space to improve the comfort level. There are many heaters having a metal housing such as baseboard, ceramic, parabolic and oil filled heaters. More recently, flame retardant polymers have been used for plastic housings and a fan provides efficient heat transfer. The use of a fan driven motor to provide a forced airflow across the heating elements allows the size of the portable heater to be reduced relative to portable heaters that operate on a natural convection cycle. When the elements are activated the fan motor typically operates at the same speed and does not appreciably vary with heat output. The fan motors are voltage controlled and designed to operate with low current. Typically, the motors are placed in parallel with the heating elements.
Examples of fan driven heaters with at least two different power outputs are disclosed in U.S. Pat. No. 4,755,653, U.S. Pat. 5,434,386, U.S. Pat. No. 5,663,633 and U.S. Pat. No. 5,245,691.
Some fan driven portable heaters also include a fan mode where no heat is produced and the device merely operates as a fan. The fan mode typically includes a switch for varying the fan speed. Most portable heaters include two or three heating elements which can be combined to produce three different heat outputs. In most cases, the heating elements are provided in a parallel configuration, however, it is possible to have one of the elements in parallel with two other elements connected in series.
A common heating circuit has the fan motor in parallel with the heating elements and operating with less than 0.5 amps. A switch is provided on the portable heater which allows the user to set the desired heat output level. In the heat generation mode, the fan motor is in parallel with the heating elements, and operates at a constant speed. Portable heaters of this type may include a separate fan speed control which is only operative when the heating elements are not functioning.
Portable heaters are designed to be connected to a 120 volts wall receptacle typically having a 15 amp current capacity. To produce 1500 watts of power this implies the current will be approximately 12.5 amps and produces approximately 5120 BTU""s of heat. This amount of heat generation requires a substantial airflow to maintain a low operating temperature with the plastic molded heater housing. A high fan speed is required at the maximum heat output and produces a significant amount of noise. As the fan runs at high speed for all heat output conditions, unnecessary noise is produced at lower heat outputs. Multispeed motors and/or resistors can be used to drop the voltage to the fan motor but significantly add to the cost of the heater and complicate the manufacturing process. These high speed, low current fans include many windings of small gauge wire which is prone to breakage and is more expensive relative to heavier gauge magnetic motor wire. As such, these motors represent a major cost component of the heater.
Various improvements have been proposed with respect to portable heaters to include additional features such as night lights or combining these heaters with other appliances such as humidifiers.
The purchase of a portable heater remains a price sensitive purchase as the heater is typically solving a specific problem requiring a temporary solution. Therefore, effective design of the portable heater is required to produce an acceptable product.
A portable heater according to the present invention comprises an electric circuit which powers a resistance heating arrangement and an electric fan and wherein the electric fan receives and is powered by a current which powers the resistance heating arrangement.
In a preferred aspect of the invention the electric fan is designed to operate at high current.
In a further aspect of the invention the electric fan is wound with 20 gauge or heavier wire.
In a further aspect of the invention the electric fan is wound with wire rated to carry at least 8 amperes and each pole of the motor has less than 50 turns.
The heating circuit of a preferred aspect of the present invention comprises a first heating element and a second heating element with a fan motor connected in series with each of these elements. A multi-position switch selectively connects a power input means with the heating elements and the fan induction motor. In a first position of the switch, the power input means is connected with the first heating element and the fan induction motor in series therewith, resulting in the fan operating at a first speed with the first heating element producing a first heat output. In a second position of the multi-position switch, the power input means is connected to the second heating element with the fan motor in series therewith and causes the fan motor to operate at a second speed greater than the first speed and the heating element produces heat at a second heat output greater than the first heat output.
With this arrangement, the fan motor speed automatically increases with increasing power of the heating elements.
Several advantages are achieved with the heating circuit of the present invention. The fan motor is easily manufactured and cooperates with the heating elements to provide effective control and safe operation. In a preferred embodiment automatic adjustment of the fan motor speed as the heat output increases or decreases effectively matches the fan motor speed with the heat output. By reducing the fan speed as the power of the heater decreases, the discharge temperature of the air as it leaves the heater is higher. Also the lower operating speed of the motor reduces motor noise and is less intrusive. The automatic adjustment of the fan motor speed is achieved by using the heating elements to vary the current to the fan motor. Thus, no additional elements have been added to the heating circuit to achieve the automatic speed adjustment of the fan.
In a preferred aspect of the invention, the multi-position switch includes a third position where the heating elements are connected in parallel with these parallel heating elements being connected in series with the fan induction motor. This results in the heating elements collectively generating heat at a third power and cause the fan to operate at a third speed.
In yet a further aspect of the invention, the fan motor is appropriately wound to pass the operating current therethrough without damage should the rotor become locked.
In yet a further aspect of the invention, the fan motor is wound with a wire of at least 20 gauge.
In yet a further aspect of the invention, the fan motor is powered with a current of at least two amps.
In yet a further aspect of the invention, the operating speed of the fan is controlled by the current provided thereto which correspondingly varies in proportion to the power output of the heating circuit.